1. Field of the Invention
This invention relates generally to cardiopulmonary support equipment, and more particularly, to non-invasive cardiopulmonary equipment which can both assist or entirely replace the natural function of a patient's heart and lungs.
2. Description of the Related Art
There are approximately 550,000 cases annually of cardiac arrest in the U.S. Despite advances in many other areas of medicine, the survival rate for these cases remains low. In general, for the victims to survive, it is essential that they receive proper resuscitation as soon as possible after the cardiac arrest. Successful cardiopulmonary support should be established within 4-5 minutes of cardiac arrest. Beyond this, any delay in providing support may result in severe brain damage.
There are two general classes of cardiopulmonary support: invasive and non-invasive. Examples of invasive support devices include percutaneous bypass, direct coronary perfusion, the Anstadt cup, hemopumps, and intraortic balloon pumping. Of course since these techniques require the insertion of devices into the body, they can only be performed by trained medical personnel. In fact, these techniques are generally not suited for emergency life support outside a hospital.
Non-invasive devices tend to be easier and less expensive to use than the invasive equipment. Non-invasive support techniques include cardiopulmonary resuscitation (CPR), leg compression, and thumper devices or compression vests which mechanically compress the chest to simulate CPR.
CPR provides cardiac support through a series of rhythmic compressions of the victim's thorax, alternating with mouth-to-mouth resuscitation. The principle advantage of CPR is its relative simplicity. An individual can be trained to administer CPR in only about 15 hours, and CPR does not require any specialized equipment.
However, CPR is not the ideal form of cardiopulmonary support; it is tiring to administer, and the thoracic compressions can severely injure a patient. Moreover, CPR is not very efficient, and ordinarily provides barely enough cardiopulmonary support to sustain the patient until professional emergency medical care can be provided.
The thumper devices and compressive vests now used for non-invasive life support have been designed to duplicate the movements used to perform CPR, the idea being to provide a mechanical substitute for a person trained to administer CPR. Examples of such devices can be found in U.S. Pa. No. 3,219,031, No. 3,509,899, No. 3,896,797, and No. 4,397,306. These patents describe devices which use reciprocable plungers to compress a victim's chest along with a means of ventilating the victim, such as a source of pressurized oxygen or a squeeze bag. However, such devices, because they are fairly complex and not easily used by untrained lay persons, are in fact less-than-ideal substitutes for a trained CPR administrator. Moreover, they suffer from the same drawbacks as manual CPR. For example, if the device is not properly positioned, the chest compressor may cause severe damage to the victim's thorax, and may not even support the cardiac function.
As an alternative to the use of mechanical chest compressors, U.S. Pat. No. 2,071,215, No. 4,424,806 and No. 4,928,674 describe how to support the pulmonary and/or cardiac functions by providing an inflatable bladder around the patient's chest. In some cases, a stiff outer shell or biasing cuff surrounds the bladder so that when the bladder is periodically inflated, the patient's chest is compressed, causing expiration and inspiration.
Because none of these devices is entirely satisfactory, CPR remains the most common resuscitative technique used by lay persons to treat cardiac arrest.
As previously noted, emergency medical personnel have available to them a number of different ways to treat cardiac arrest. However, none of these techniques are entirely satisfactory. Thus, there is a need for a CPR resuscitation device which is simple, easy to use, and not harmful to patients.
If the patient's heart is weakened, but still beating, it may be beneficial to reduce the burden on the heart. If cardiac assistance is rendered to a conscious patient, it is also important that this be done in a way which does not unnecessarily discomfort the patient.
Therefore, there is also a need for a device which can assist a beating heart even while the patient is conscious. It is also desirable that such assistance not unduly discomfort the patient.
A patient suffering cardiac arrest also requires pulmonary support, since breathing stops during cardiac failure. Accordingly, a cardiopulmonary support device must also in some way provide a substitute for breathing.
Heretofore, this has been accomplished by ventilating the patient's lungs using a source of pressurized air or oxygen. Typically, this was done by intubating the patient and then filling the lungs with fresh air or oxygen at approximately the normal breathing frequency.
A recent study determined that where cardiac support is provided by rhythmic chest compressions, cardiac output can be significantly improved by alternating chest compressions with chest decompressions. In this study, the chest was compressed and decompressed using a rubber plunger, which alternately applied pressure and suction to the patient's chest. See Cohen, T. J., et al., "Active Compression-Decompression: A New Method of Cardiopulmonary Resuscitation", J. Am. Med. Assoc. Vol. 267, No. 21, pp. 2916-23, 1992. This technique is known as active compression-decompression CPR ("ACD CPR").
In tests, when a 30 lb. anterior force was applied to the thorax of an adult male, blood pressure measured by a finger plethysmograph was increased by approximately 10 mm Hg in response thereto. Then, when a 30 lb. lateral force was applied, the blood pressure dropped by approximately 16 mm Hg.
Cadaver tests have confirmed the effectiveness of ACD CPR. Anterior compression of the cadaver thorax increased aortic blood pressure by approximately 76 mm Hg. Lateral compression of the thorax decreased blood pressure by approximately 12 mm Hg, as did anterior suction of the thorax.
ACD CPR is significantly more effective than conventional "compression-only" CPR. It provides both perfusion and ventilation, and can resuscitate patients where conventional CPR and defibrillation fail. However, the plunger used to decompress the chest introduces a number of complications. For example, since decompression requires a vacuum be maintained between the plunger and the chest, it is imperative to provide a good seal between the plunger and the chest. Moreover, since people of all ages suffer cardiac arrest, it is necessary to be able to treat people of all sizes. Accordingly, a range of different size and shape plungers may be needed. Even then, if the patient is hirsute, heavily-muscled, or large-breasted, it may prove impossible to provide a seal which allows adequate decompression.
Thus, there exists a need for a support device which compresses and decompresses the patient's chest without using a plunger, and which device is sufficiently flexible to accommodate a wide range of victim body types.